running extended plenums side by side

right now we have three 8x8 extended plenums for the supply that come off one main takeoff from the furnace plenum, they have them spaced out a foot apart, which takes a ton of room up, is it possible to run them side by side with no real gap between them to save space on the ceiling? if so how do you do something like this? and how would you anchor them up? thanks!